The invention relates to a process for enriching the presence of H. pylori NAP in a mixture of proteins. The process involves salting-out other proteins. NAP has been found to remain soluble at ammonium sulphate concentrations of 80% and above. The process may involve the additional step of metal chelate chromatography. The combination of salting-out and metal chelate chromatography results in very pure NAP. The NAP may have the same sequence as NAP naturally occurring in H. pylori and is free from sequences typically associated with recombinant protein production. The processes and NAP of the invention can be used in diagnostic and therapeutic products and processes.